A direct-injection system comprises a number of injectors; a common rail, which feeds pressurized fuel to the injectors; a high-pressure pump, which feeds fuel to the common rail along a feed line, and has a flow regulating device; and a control unit, which controls the flow regulating device to maintain a desired fuel pressure in the common rail, which normally varies as a function of engine operating conditions.
The high-pressure pump comprises at least one pumping chamber, in which a piston slides back and forth; an intake pipe regulated by an intake valve to feed low-pressure fuel to the pumping chamber; and a delivery pipe regulated by a delivery valve to feed high-pressure fuel from the pumping chamber along the feed line to the common rail. The flow regulating device normally acts on the intake valve to also keep it open during the pumping stage, so that a varying amount of fuel in the pumping chamber flows back into the intake pipe, as opposed to being pumped along the feed line to the common rail.
It has recently been proposed to form a drain channel in the high-pressure pump, connecting the delivery pipe to the pumping chamber, and regulated by a one-way maximum-pressure valve, which only allows fuel flow from the delivery pipe to the pumping chamber, and serves as a fuel bleed valve, in the event the fuel in the common rail exceeds a maximum design pressure (typically as a result of control errors by the control unit). In other words, the maximum-pressure valve is calibrated to open automatically when the difference between the pressures on either side of it exceeds a design threshold value, and so prevent the fuel in the common rail from exceeding the maximum design pressure.
The maximum-pressure valve normally comprises a ball shutter movable along the drain channel; and a valve seat engaged in fluidtight manner by the shutter. A calibrated spring pushes the shutter into a position engaging the valve seat in fluidtight manner; and the elastic pressure of the spring is calibrated so the shutter only detaches from the valve seat when the difference between the pressures on either side of the maximum-pressure valve exceeds the design threshold value.
Fuel flow along the drain channel, when the maximum-pressure valve opens, varies, depending on engine speed, i.e. depending on flow from the high-pressure pump, the actuating frequency of which is directly proportional to engine speed. In other words, in the event of a high flow rate from the high-pressure pump, it feeds a large amount of fuel to the common rail, and, if the fuel pressure in the common rail is too high, a correspondingly large amount of fuel must be drained from the common rail along the drain channel.
For a large amount of fuel to flow along the drain channel, the maximum-pressure valve needs a large flow opening, which means the shutter must move a good distance away from the valve seat, thus exerting greater pressure on the spring. Conversely, for a small amount of fuel to flow along the drain channel, the maximum-pressure valve only needs a small flow opening, which means the shutter need only move a small distance away from the valve seat, thus exerting less pressure on the spring. In other words, an increase in fuel flow along the drain channel calls for a proportional increase in the size of the flow opening of the maximum-pressure valve, and therefore a proportional increase in the movement of the shutter, greater pressure on the spring, and greater elastic pressure by the spring on the shutter. The increase in the elastic pressure of the spring on the shutter inevitably calls for greater fuel pressure in the common rail, since, to keep the valve open, the hydraulic pressure exerted on the shutter by the fuel pressure must equal the elastic pressure exerted on the shutter by the spring.
Put briefly, at low engine speed (i.e. with a low flow rate from the high-pressure pump), the maximum fuel pressure in the common rail is lower, whereas, at high engine speed (i.e. with a high flow rate from the high-pressure pump), the maximum fuel pressure in the common rail is higher. The increase in the maximum fuel pressure in the common rail alongside an increase in engine speed is by no means negligible, and may even be as much as 50% of the maximum fuel pressure at idling speed.
To allow for the increase in maximum fuel pressure in the common rail alongside an increase in engine speed, all the component parts (pipes, common rail, pressure sensor, and above all the injectors) must be designed to safely withstand the maximum possible fuel pressure in the common rail, despite this affording no advantages in terms of operation. Oversizing the component parts affected by the increase in maximum fuel pressure in the common rail alongside an increase in engine speed obviously means a considerable increase in cost and weight (stronger components are necessarily heavier), that affords no functional advantage.
US2007286742A1 discloses a direct-injection system fuel pump having: at least one pumping chamber; a piston mounted to slide inside the pumping chamber to cyclically alter the volume of the pumping chamber; an intake channel connected to the pumping chamber and regulated by an intake valve; a delivery channel connected to the pumping chamber and regulated by a one-way delivery valve that only permits fuel flow from the pumping chamber; and a drain channel regulated by a one-way, maximum-pressure valve, which opens when the fuel pressure in the drain channel exceeds a threshold value, and which has a shutter movable along the drain channel, a valve seat engaged in fluidtight manner by the shutter, and a spring calibrated to push the shutter into a position engaging the valve seat in fluidtight manner.